feat(files): gate files. browse behind FILES_BROWSE (on|off)

Per-host toggle for the read-only, no-auth browse of the shared upload dir,
so it can stay on for the LAN-party host (oxps) and be off on the
internet-facing one (cochi). Defaults to `on` — existing hosts unchanged.

Neither layer has conditionals, so each gets the value substituted in:
- caddy: docker-compose mounts 62-files.${FILES_BROWSE:-on}.caddy at the
  fixed conf.d target (62-files.caddy -> 62-files.on.caddy, plus a
  comments-only 62-files.off.caddy stub — caddy needs a parseable file
  there). Same trick as FILES_MOUNT_MODE.
- nginx: render-nginx.sh splices nginx/files-browse.server.tmpl into
  ${FILES_BROWSE_SERVER} and adds/omits the name in the port-80 redirect.
  This is the gate that matters; caddy is bound to 127.0.0.1.

Applying a change needs render-nginx.sh --install plus a caddy
--force-recreate (single-file bind mount pins the inode). With `off`,
drop `files` from LE_SUBDOMAINS on that host. upload. is unaffected.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

nginx/files-browse.server.tmpl

@@ -0,0 +1,24 @@
+# Read-only static browse of the same share (files.) — caddy serves the dir
+# listing, no auth. Read-only, so no upload tuning: standard 4m body is fine.
+#
+# Rendered into ulicraft-caddy.conf.tmpl at ${FILES_BROWSE_SERVER} by
+# render-nginx.sh, ONLY when FILES_BROWSE=on. With `off` the placeholder
+# collapses to a comment, so the name has no TLS vhost at all (and needs no
+# cert — drop `files` from LE_SUBDOMAINS on that host).
+server {
+    listen 443 ssl http2;
+    listen [::]:443 ssl http2;
+    server_name files.${BASE_DOMAIN};
+
+    ssl_certificate     ${APP_DIR}/certs/files.${BASE_DOMAIN}/cert.pem;
+    ssl_certificate_key ${APP_DIR}/certs/files.${BASE_DOMAIN}/key.pem;
+    add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;
+
+    location / {
+        proxy_pass http://ulicraft_caddy;
+        proxy_set_header Host              $host;
+        proxy_set_header X-Real-IP         $remote_addr;
+        proxy_set_header X-Forwarded-For   $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+    }
+}